Nonvolatile semiconductor memory device

ABSTRACT

According to one embodiment, in a nonvolatile semiconductor memory device, a first line is disposed on a semiconductor substrate. A first memory cell is disposed on a side opposite to the semiconductor substrate with respect to the first line. A second line intersects with the first line via the first memory cell. A second memory cell is disposed on a side opposite to the semiconductor substrate with respect to the second line. A third line intersects with the second line via the second memory cell. The first memory cell has a first resistance change layer and a first rectification layer. The second memory cell has a second resistance change layer and a second rectification layer. A composition of the first resistance change layer is different from a composition of the second resistance change layer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority fromJapanese Patent Application No. 2011-066672, filed on Mar. 24, 2011; theentire contents of which are incorporated herein by reference.

FIELD

Embodiments described herein relate generally to a nonvolatilesemiconductor memory device.

BACKGROUND

In recent years, there has been a demand for a nonvolatile memory whichdoes not depend on transistor operation. As the nonvolatile memory,since a resistance change memory such as a phase change memory (PCM)element and a resistance RAM (ReRAM) element utilizes a resistancechange state of a resistance material for operation, the transistoroperation is unnecessary for write/erase. Also, write/erase speed islowered or a write/erase current is increased due to difficulty incompletely changing a resistance of the resistance change material inthe case where the size of the resistance material is large (100 nm to 1μm), but it is easy to completely change the resistance of theresistance change material by miniaturizing the size of the resistancematerial. Therefore, the resistance change memory has thecharacteristics that element properties are improved along with theminiaturization to thereby realize write/erase speed improvement andpower consumption reduction. Also, the resistance change memory having amultilayer structure has the advantage that a bit density thereof isrelatively easily increased. Therefore, along with an increase insemiconductor products which require high capacity data processing,there is an increasing need for the resistance change memory.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are diagrams showing a configuration of a nonvolatilesemiconductor memory device according to a first embodiment;

FIGS. 2A and 2B are diagrams showing the configuration of thenonvolatile semiconductor memory device according to the firstembodiment;

FIGS. 3A and 3B are diagrams showing a method for producing thenonvolatile semiconductor memory device according to the firstembodiment;

FIGS. 4A and 4B are diagrams showing the method for producing thenonvolatile semiconductor memory device according to the firstembodiment;

FIGS. 5A and 5B are diagrams showing the method for producing thenonvolatile semiconductor memory device according to the firstembodiment;

FIG. 6 is a diagram showing a relationship between a film thickness of aresistance change layer and data retention according to the firstembodiment;

FIGS. 7A and 7B are diagrams showing a configuration of a nonvolatilesemiconductor memory device according to a second embodiment;

FIG. 8 is a diagram showing a relationship between a composition of aresistance change layer and data retention according to the secondembodiment;

FIGS. 9A and 9B are diagrams showing a configuration of a nonvolatilesemiconductor memory device according to a third embodiment;

FIGS. 10A and 10B are diagrams showing the configuration of thenonvolatile semiconductor memory device according to the thirdembodiment;

FIGS. 11A and 11B are diagrams showing a method for producing thenonvolatile semiconductor memory device according to the thirdembodiment;

FIGS. 12A and 12B are diagrams showing a method for producing thenonvolatile semiconductor memory device according to the thirdembodiment; and

FIGS. 13A and 13B are diagrams showing a relationship between a cellsize (a width of a resistance change layer) and data retention.

DETAILED DESCRIPTION

In general, according to one embodiment, there is provided a nonvolatilesemiconductor memory device including a semiconductor substrate, a firstline, a first memory cell, a second line, a second memory cell, and athird line. The first line is disposed on the semiconductor substrate.The first memory cell is disposed on a side opposite to thesemiconductor substrate with respect to the first line. The second lineintersects with the first line via the first memory cell. The secondmemory cell is disposed on a side opposite to the semiconductorsubstrate with respect to the second line. The third line intersectswith the second line via the second memory cell. The first memory cellhas a first resistance change layer and a first rectification layer. Thesecond memory cell has a second resistance change layer and a secondrectification layer. A composition of the first resistance change layeris different from a composition of the second resistance change layer.

Exemplary embodiments of a nonvolatile semiconductor memory device willbe explained below in detail with reference to the accompanyingdrawings. The present invention is not limited to the followingembodiments.

First Embodiment

A nonvolatile semiconductor memory device 100 according to the firstembodiment will be described by using FIGS. 1A and 1B. FIG. 1A is aperspective view schematically showing a configuration of thenonvolatile semiconductor memory device 100, and FIG. 1B is anequivalent circuit diagram of a portion including an upper memory celland a lower memory cell in the nonvolatile semiconductor memory device.

In the nonvolatile semiconductor memory device 100, a wiring layer and amemory layer are alternately layered for plural times on a semiconductorsubstrate SB. For example, as shown in FIG. 1A, a wiring layer WL1, amemory layer MC1, a wiring layer BL1, a memory layer MC2, a wiring layerWL2, a memory layer MC3, a wiring layer BL2, a memory layer MC4, and awiring layer WL3 are layered in a direction Z in this order. As usedherein, “on the semiconductor substrate SB” is an expression includingnot only the case of being disposed directly on the semiconductorsubstrate SB but also the case of being disposed on the semiconductorsubstrate SB via a predetermined layer (e.g. interlayer insulation filmor the like).

Each of the wiring layers WL1, WL2, WL3 extends in a direction X and hasplural word lines aligned in a direction Y. For example, in the wiringlayer WL1, the plural word lines WL11 to WL13 extend in the direction Xand are aligned in the direction Y. For example, in the wiring layerWL2, the plural word lines WL21 to WL23 extend in the direction X andare aligned in the direction Y. In the wiring layer WL3, the plural wordlines WL31 to WL33 extend in the direction X and are aligned in thedirection Y.

Each of the wiring layers BL1 and BL2 extends in the direction Y and hasplural bit lines aligned in the direction X. For example, in the wiringlayer BL1, the plural bit lines BL 11 to BL 13 extend in the direction Yand are aligned in the direction X. For example, in the wiring layerBL2, the plural bit lines BL 21 to BL 23 extend in the direction Y andare aligned in the direction X.

Each of the memory layers MC1, MC2, MC 3, and MC4 is disposed betweenthe wiring layer including the word lines and the wiring layer includingthe bit lines. For example, the memory layer MC1 is disposed between thewiring layer WL1 and the wiring layer BL1. For example, the memory layerMC2 is disposed between the wiring layer BL1 and the wiring layer WL2.For example, the memory layer MC3 is disposed between the wiring layerWL2 and the wiring layer BL2. For example, the memory layer MC4 isdisposed between the wiring layer BL2 and the wiring layer WL3.

Also, each of the memory layers MC1, MC2, MC3, and MC4 has plural memorycells disposed at plural intersections between the plural word lines andthe plural bit lines (i.e. disposed in matrix). For example, in thememory layer MC1, the plural memory cells MC111 to MC133 are disposed atplural intersections between the plural word liens WL11 to WL13 and theplural bit lines BL11 to BL13, i.e. disposed in matrix. For example, inthe memory layer MC2, the plural memory cells MC211 to MC233 aredisposed at plural intersections between the plural bit lines BL11 toBL13 and the plural word liens WL21 to WL23, i.e. disposed in matrix.For example, in the memory layer MC3, the plural memory cells MC311 toMC333 are disposed at plural intersections between the plural word liensWL21 to WL23 and the plural bit lines BL21 to BL23, i.e. disposed inmatrix. For example, in the memory layer MC4, the plural memory cellsMC411 to MC433 are disposed at plural intersections between the pluralbit lines BL21 to BL23 and the plural word liens WL31 to WL33, i.e.disposed in matrix.

As viewed in the direction Z, for example, the memory cell MC111 isdisposed on the word line WL11. The hit line BL11 intersects the wordline WL11 via the memory cell MC111. The memory cell MC211 is disposedon the bit line BL11. The word line WL21 intersects the word line WL21via the memory cell MC211. An equivalent circuit of this portion isshown in FIG. 1B.

As shown in FIG. 1B, in the memory cell MC111 (lower memory cell)disposed at the intersection between the word line WL11 and the bit lineBL11, a rectification element D111 and a resistance change element R111are serially connected. The rectification element D111 is a diode, forexample, in which a cathode is connected to the resistance changeelement R111, and an anode is connected to the word line WL11. One endof the resistance change element R111 is connected to the bit line BL11,and the other end is connected to the rectification element D111.

Also, in the memory cell MC211 (upper memory cell) disposed at theintersection between the bit line BL11 and the word line WL21, arectification element D211 and a resistance change element R211 areserially connected. The rectification element D211 is a diode, forexample, in which a cathode is connected to the bit line BL11, and ananode is connected to the resistance change element R211. One end of theresistance change element R211 is connected to the word line WL21, andthe other end is connected to the rectification element D211.

As described above, the nonvolatile semiconductor memory device 100 maybe a resistance change memory of a crosspoint type, for example.

Though the configuration in which the direction from the word line tothe bit line is a forward direction of the rectification elements D111,D211 is exemplified in FIG. 1B as a configuration of each of the memorycells MC111, MC211, a configuration in which the direction from the wordline to the bit line is a backward direction of the rectificationelements D111, D211 may be used. Also, in each of the memory cellsMC111, MC211, the positions of the rectification element and theresistance change element may be interchanged with each other.

Hereinafter, each of layer structures of the lower memory cell MC111 andthe upper memory cell MC211 shown in FIG. 1B will be described by usingFIGS. 2A and 2B. FIG. 2A is a diagram showing a sectional structure inthe case where the nonvolatile semiconductor memory device 100 is cutalong a plane which is perpendicular to the direction X, and FIG. 2A isa diagram showing a sectional structure in the case where thenonvolatile semiconductor memory device 100 is cut along a plane whichis perpendicular to the direction Y.

The lower memory cell MC111 shown in FIG. 1B is disposed at a positionwhere a line pattern 102 corresponding to the word line WL11 and a linepattern 110 corresponding to the bit line BL11 intersect with each otheras shown in FIGS. 2A and 2B. The memory cell MC111 is insulated from theadjacent memory cell in the same memory layer MC1 by an interlayerinsulation film 109.

In the lower memory cell MC111, a barrier metal layer 103, a diode layer(rectification layer) 104, a lower electrode layer 105, a resistancechange layer 106, an upper electrode layer 107, and a CMP stopper layer108 are layered in this order.

The harrier metal layer 103 is disposed on the line pattern 102. Thebarrier metal layer 103 is formed from a conductor such as a metal (e.g.titanium nitride).

The diode layer 104 is disposed on the barrier metal layer 103. Thediode layer 104 has a Metal-Insulator-Metal (MIM) structure, a P+polySilicon-Intrinsic-N+poly Silicon (PIN) structure, or the like. In thecase of the PIN structure, the diode layer 104 has a structure in whichan N-type layer, an I-type layer, and a P-type layer are layered. TheN-type layer is formed from a semiconductor (e.g. silicon) containingN-type impurities such as arsenic and phosphor. The I type layer isformed from a so-called intrinsic semiconductor (e.g. silicon) whichdoes not contain any impurity. The P-type layer is formed from asemiconductor (e.g. silicon) containing P-type impurities such as boron.The diode layer 104 functions as the rectification element D111 in thememory cell MC111 (see FIG. 1B).

The lower electrode layer 105 is disposed on the diode layer 104. Thelower electrode layer 105 is formed from a conductor such as a metal(e.g. titanium nitride). The lower electrode layer 105 functions as alower electrode to the resistance change layer 106.

The resistance change layer 106 is disposed on the lower electrode layer105. The resistance change layer 106 is formed from a material selectedfrom the group consisting of ZnMn₂O₄, NiO, HfO, TiO₂, SrZrO₃, andPr_(0.7)Ca_(0.3)MnO₃, for example. The resistance change layer 106functions as the resistance change element 8111 in the memory cell MC111(see FIG. 1B).

The upper electrode layer 107 is disposed on the resistance change layer106. The upper electrode layer 107 is formed from a conductor such as ametal (e.g. titanium nitride). The upper electrode layer 107 functionsas an upper electrode for the resistance change layer 106.

The CMP stopper layer 108 is disposed on the upper electrode layer 107.The CMP stopper layer 108 is formed from a conductor such as a metal(e.g. tungsten). The CMP stopper layer 108 functions as a stopper filmwhen an upper surface is flattened in a production step as describedlater in this specification.

As shown in FIGS. 2A and 2B, the upper memory cell MC211 shown in FIG.1B is disposed at a position where a line pattern 110 corresponding tothe bit line BL11 and a line pattern 133 corresponding to the word lineWL21 intersect with each other. Also, the memory cell MC211 is insulatedfrom the adjacent memory cell in the same memory layer MC2 via aninterlayer insulation film 134.

In the upper memory cell MC211, a barrier metal layer 111, a diode layer(rectification layer) 112, a lower electrode layer 113, a resistancechange layer 114, an upper electrode layer 115, and a CMP stopper layer116 are layered in this order.

Structures of the layers in the upper memory cell MC211 are basicallythe same as the lower memory cell MC111 and are different from the lowermemory cell MC111 by the following features. A film thickness D₁₀₆ ofthe resistance change layer 106 in the lower memory cell MC111 isthicker than a film thickness D₁₁₄ of the resistance change layer 114 inthe upper memory cell MC211. More specifically, the film thickness D₁₀₆is thicker than the film thickness D₁₁₄ for being decided by consideringthat the number of heating processes performed for forming the diodelayer in the lower memory cell MC111 is larger than that performed inthe upper memory cell MC211. For example, the film thickness D₁₁₄ may beabout 5 nm, and the film thickness B₁₀₆ may be about 10 nm.

The upper memory cell MC211 and the lower memory cell MC11 are disposedon a base film 101.

Hereinafter, a method for producing the nonvolatile semiconductor memorydevice 100 will be described by using FIGS. 3 to 5 and FIGS. 2A and 2B.FIGS. 3A to 5B are sectional views showing steps of the method forproducing the nonvolatile semiconductor memory device 100. FIGS. 2A and2B includes the diagrams showing sectional structures of the nonvolatilesemiconductor memory 100 and are used as sectional views showing thesteps of the production method.

In the step shown in FIG. 3A, on the base film 101 including thesemiconductor substrate SB (or the semiconductor substrate SB and theinterlayer insulation film formed thereon and the like), a stacked filmSF1 in which a conduction layer 102 a, a conduction layer 103 a, a diodelayer 104 a, a conduction layer 105 a, a resistance change layer 106 a,a conduction layer 107 a, and a CMP stopper layer 108 a are layered inthis order is formed.

For example, in the case where the diode layer 104 a has the PINstructure, P-type impurities are introduced into a semiconductor layerwhich is to serve as a P-type layer in the diode layer 104 a in-situ orafter formation, and then a heat treatment for activating the P-typeimpurities is performed. Also, N-type impurities are introduced into asemiconductor layer which is to serve as an N-type layer in the diodelayer 104 a in-situ or after formation, and then a heat treatment foractivating the N-type impurities is performed.

In the step shown in FIG. 3B, the stacked film SF1 is etched into pluralfin-like members FIN1 which extend in the direction X and aligned in thedirection Y. Thus, the conduction layer 102 a is divided into pluralline patterns 102 corresponding to the word line WL11 and the like. Ineach of the fin-like members FIN1, the line pattern 102, a conductionlayer 103 b, a diode layer 104 b, a conduction layer 105 b, a resistancechange layer 106 b, a conduction layer 107 b, and a CMP stopper layer108 b are layered in this order (see FIG. 4B). Next, an interlayerinsulation film 109 is embedded between the adjacent fin-like membersFIN 1, followed by flattening by CMP. Here, since the CMP stopper layer108 b serves as a stopper, the upper surface is easily flattened.

In steps shown in FIGS. 4A and 4B, a stacked film SF2 in which aconduction layer 110 a, a conduction layer 111 a, a diode layer 112 a, aconduction layer 113 a, a resistance change layer 114 a, a conductionlayer 115 a, and a CMP stopper layer 116 a are layered in this order isformed.

For example, in the case where the diode layer 112 a has the PINstructure, P-type impurities are introduced into a semiconductor layerwhich is to serve as a P-type layer in the diode layer 112 a in-situ orafter formation, and then a heat treatment for activating the P-typeimpurities is performed. Also, N-type impurities are introduced into asemiconductor layer which is to serve as an N-type layer in the diodelayer 112 a in-situ or after formation, and then a heat treatment foractivating the N-type impurities is performed.

In the step shown in FIGS. 5A and 5B, the stacked film SF2 is etchedinto plural fin-like members FIN2 which extend in the direction Y andaligned in the direction X. Thus, the conduction layer 110 a is dividedinto plural line patterns 110 in accordance with the bit line BL11 andthe like. In each of the fin-like members FIN2, the line pattern 110, aconduction layer 111 b, a diode layer 112 b, a conduction layer 113 b, aresistance change layer 114 b, a conduction layer 115 b, and a CMPstopper layer 116 b are layered in this order.

Simultaneously, the plural lower fin-like members FIN1 are etched intoplural memory cells which are two-dimensionally aligned in the directionX and the direction Y. More specifically, a memory cell array in whichthe plural memory cells MC111 to MC 133 in the lower memory layer MC1are aligned in a matrix is formed (see FIG. 1A).

Next, an interlayer insulation film 134 is embedded between the adjacentfin-like members FIN2, followed by flattening by CMP. Here, since theCMP stopper layer 116 b serves as a stopper, the upper surface is easilyflattened.

In the steps shown in FIGS. 2A and 2B, the plural upper fin-like membersFIN2 are etched into plural memory cells which are two-dimensionallyaligned in the direction X and the direction Y. More specifically, amemory cell array in which the plural memory cells MC211 to MC 233 inthe upper memory layer MC2 are aligned in matrix is formed (see FIG.1A). The above-described steps are repeated to form memory cell arraysfor three layers or more.

Also, in order to clarify the effect according to the first embodiment,results of evaluation conducted on a relationship between a filmthickness of a resistance change layer and data retention will bedescribed by using FIG. 6.

As shown in FIG. 6, three different memory cell arrays were prepared.More specifically, the three types of memory cell arrays in each ofwhich memory cells including resistance change layers having a filmthickness of D1, D2, or D3 were prepared, and data retention aftersetting (after lowering resistance) was evaluated.

More specifically, the film thicknesses D1, D2, and D3 were decided tosatisfy

D1<D2<D3

within a range of 5 to 15 nm. After an elapse of a predetermined timeperiod, a proportion of bits (memory cells) having good data retentionwas evaluated in each of the memory cell arrays. As a result, it wasconfirmed that there is a tendency that the proportion of bits havinggood data retention is increased along with the film thickness increaseof D1<D2<D3 of the resistance change layer. From the findings, it wasconfirmed that a fluctuation in data retention between the upper memorycell and the lower memory cell is reduced when the film thickness D₁₀₆of the resistance change layer 106 in the lower memory cell MC111 inwhich the larger number of heating processes is performed is larger thanthat of the film thickness D₁₁₄ of the resistance change layer 114 inthe upper memory cell MC211 in which the smaller number of heatingprocesses is performed.

Hereinafter, as a comparative case, a case in which the film thicknessof the resistance change layer of the upper memory cell and the filmthickness of the resistance change layer of the lower memory cell aresubstantially the same will be investigated. In this case, for example,in the above-described production method, the number of heatingprocesses for forming the diode layer is 2 in the memory layer MC2including the upper memory cell MC211, while the number of heatingprocesses is 4 in the memory layer MC1 including the lower memory cellMC111. Thus, as compared to the resistance change layer of the uppermemory cell MC211, the data retention tends to be deteriorated since theresistance change layer of the lower memory cell MC111 is exposed to thelarger number of heating processes. Therefore, there is a tendency thata fluctuation in data retention occurs between the upper memory cell andthe lower memory cell.

In contrast, in the first embodiment, the film thickness of theresistance change layer in the lower memory cell is increased to belarger than the film thickness of the resistance change layer in theupper memory cell. With such configuration, since the fluctuation indata retention due to the difference in number of heating processes isreduced, it is possible to reduce the fluctuation in data retentionbetween the upper memory cell and the lower memory cell.

It should be noted that, in the nonvolatile semiconductor memory device100, the film thickness of the resistance change layer of the memorycell in the memory layer MC1 may be thicker than the film thickness ofthe resistance change layer of the memory cells in the memory layersMC2, MC3, and MC4. Alternatively, the film thicknesses of the resistancechange layers of the memory cells of the memory cells MC1 and MC2 may besubstantially equal to each other and thicker than the film thicknessesof the memory cells of the memory layers MC3 and MC4. Alternatively, thefilm thicknesses of the resistance change layers of the memory cells ofthe memory layers MC1, MC2, and MC3 may be substantially equal to eachother and thicker than the film thickness of the resistance change layerof the memory cell in the memory layer MC4.

Alternatively, the film thicknesses of the resistance layers in thenonvolatile semiconductor memory device 100 may be thickened stepwisefrom the memory layer of the upper layer to the memory layer of thelower layer. For example, in the configuration shown in FIG. 1A, filmthickness of resistance layer of memory cell in memory cell MC4<filmthickness of resistance layer of memory cell in memory cell MC3<filmthickness of resistance layer of memory cell in memory cell MC2<filmthickness of resistance layer of memory cell in memory cell MC1 may besatisfied.

Thus, it is possible to reduce in a stepwise manner the fluctuation indata retention which is caused by heat history differences among diodeformations in the layers.

Second Embodiment

Hereinafter, a nonvolatile semiconductor memory device 100 i accordingto the second embodiment will be described. A feature which is differentfrom the first embodiment will be mainly described in the following.

The second embodiment is different from the first embodiment by thefeature that a composition of a resistance change layer of a memory cellin a lower memory layer is different from a composition of a resistancechange layer of a memory cell in an upper memory layer.

More specifically, as shown in FIGS. 7A and 7B, in the nonvolatilesemiconductor memory device 100 i, a composition of a resistance changelayer 106 i which functions as a resistance change element RIM in alower memory cell MC111 i is reduced in degree of deterioration in dataretention caused by heat as compared to a resistance change layer 114 iwhich functions as a resistance change element R211 i in an upper memorycell MC211 i. For example, the resistance change layer 106 i of thelower memory cell MC111 i is formed from a carbon-based material, andthe resistance change layer 114 i in the upper memory cell MC211 i ismade from a metal oxide.

Also, in order to clarify the effect according to the second embodiment,results of evaluation conducted on a relationship between a compositionof a resistance change layer and data retention will be described byusing FIG. 8.

As shown in FIG. 8, three different memory cell arrays were prepared.More specifically, the three types of memory cell arrays in each ofwhich plural memory cells including resistance change layers of a metaloxide, a first carbon-based material, or a second carbon-based materialwere prepared, and data retention after setting (after loweringresistance) was evaluated.

More specifically, probability distribution of a current Iset aftersetting (after lowering resistance) and a current Ireset after resetting(after increasing resistance) was obtained after a predetermined timehad passed, and a value at which Iset and Ireset intersect with eachother was detected. The results are shown in FIG. 8. The results in FIG.8 indicate that the larger the value is, the better the data retentionbecomes. More specifically, it was confirmed that there is a tendencythat a proportion of bits having good data retention is increased alongwith the change of the composition of the resistance change layer ofmetal oxide→first carbon-based material→second carbon based material.From the findings, it was confirmed that the fluctuation in dataretention between the upper memory cell and the lower memory cell isreduced by forming the resistance change layer 106 i in the lower memorycell MC111 i to which the larger number of heating processes isperformed by using the carbon-based material and forming the resistancechange layer 114 i in the upper memory cell MC211 i to which the smallernumber of heating processes is performed by using the metal oxide.

The metal oxide is a material containing at least one selected from thegroup consisting of ZnMn₂O₄, NiO, HfO, TiO₂, SrZrO₃, andPr_(0.7)Ca_(0.3)MnO₃ as a main component, for example. The firstcarbon-based material is a material containing SiC as a main component,for example. The second carbon-based material is a material containing aC nanotube as a main component, for example.

As described above, in the second embodiment, the composition of theresistance change layer in the lower memory cell has a smaller degree ofdeterioration in data retention due to heat than that of the compositionof the resistance change layer in the upper memory cell. With suchconfiguration, it is possible to reduce the fluctuation in dataretention between the upper memory cell and the lower memory cell byreducing the fluctuation in data retention which is otherwise caused bythe difference in number of heating processes.

For example, the resistance change layer in the lower memory cell isformed from the carbon-based material, and the resistance change layerin the upper memory cell is formed from the metal oxide. With suchconfiguration, the degree of deterioration in data retention due to heatof the composition of the resistance change layer in the lower memorycell is smaller than that of the composition of the resistance changelayer in the upper memory cell.

In the nonvolatile semiconductor memory device 100 i, the compositionsof the resistance change layers may be changed stepwise from the uppermemory layer to the lower memory layer. For example, in theconfiguration shown in FIG. 1A, the resistance change layers of thememory cells in the memory layers MC3 and MC4 may be formed from themetal oxide; the resistance change layers of the memory cells in thememory layer MC2 may be formed from the first carbon-based material; andthe resistance change layers of the memory cells in the memory layer MC1may be formed from the second carbon-based material.

With such configuration, it is possible to reduce in the stepwise mannerthe fluctuation in data retention due to heat history differences amongdiode formations in the layers.

Alternatively, the first embodiment and the second embodiment may becombined. More specifically, a film thickness of the resistance layer inthe lower memory cell is increased to be larger than a film thickness ofthe resistance change layer in the upper memory cell, and thecomposition having the smaller degree of deterioration in data retentiondue to heat than that of the composition of the resistance change layerin the upper memory cell is used for the resistance change layer in thelower memory cell.

Third Embodiment

Hereinafter, a nonvolatile semiconductor memory device 100 j accordingto the third embodiment will be described. A feature which is differentfrom the first embodiment will be mainly described in the following.

The third embodiment is different from the first embodiment by thefeature that a width in a planar direction of a resistance change layerof a memory cell in a lower memory layer is wider than a width in aplanar direction of a resistance change layer of a memory cell in anupper memory layer.

More specifically, the nonvolatile semiconductor memory device 100 j isobtainable by modifications of adding an insulation film 192 (see FIGS.10A and 10B) between a wiring layer BL1 and a wiring layer WL2 byeliminating a memory layer MC2 and widening a line width of word linesin the wiring layer WL1 and bit lines in the wiring layer BL1.

For example, as shown in FIGS. 9A and 9B, a line width W1 of word linesWL11 j, WL12 j in a wiring layer WL1 j is wider than a line width W3 ofword lines WL21 j to WL24 j in a wiring layer WL2 j. The line width W1may be equivalent to a sum of a value which is a double of the linewidth W3 and a space between the word lines WL21 i and WL22 j. Also, aline width W2 of bit lines BL11 j, BL12 j in a wiring layer BL1 j iswider than a line width W4 of bit lines BL21 j to BL24 j in wiring layerBL2 j. The line width W2 may be equivalent to a sum of a value which isa double of the line width W4 and a space between the bit lines BL21 iand BL22 j.

Accordingly, a width in a planar direction of a lower memory cell MC111j (see FIG. 9B) positioned on an intersection between the word line WL11j and the bit line BL11 j is wider than a width in a planar direction ofan upper memory cell MC311 j (see FIG. 9A) positioned on an intersectionbetween the word line WL21 j and the bit line BL21 j.

More specifically, as shown in FIGS. 10A and 10B, in the memory cellMC111 j, each of a barrier metal layer 174, a diode layer (rectificationlayer) 175, a lower electrode layer 176, a resistance change layer 177,an upper electrode layer 178, and a CMP stopper layer 179 a lower memorycell MC111 shown in FIG. 1B has a width corresponding to the line widthW1 of a line pattern 173 corresponding to the word line WL11 j and awidth corresponding to the line width W2 of a line pattern 181corresponding to the bit line BL11 j as shown in FIGS. 2A and 2B. Forexample, the resistance change layer 177 has a width W177 in thedirection Y corresponding to the line width W1 and a width W206 in thedirection X corresponding to the line width W2. The diode layer 175functions as a rectification element D111 j, and the resistance changelayer 177 functions as a resistance change element R111 j.

In contrast, in the upper memory cell MC311 j, each of a barrier metallayer 194, a diode layer (rectification layer) 195, a lower electrodelayer 196, a resistance change layer 197, an upper electrode layer 198,and a CMP stopper layer 199 has a width corresponding to the line widthW1 of a line pattern 193 corresponding to the word line WL21 j and awidth corresponding to the line width W2 of a line pattern 220corresponding to the bit line BL21 j. For example, the resistance changelayer 197 has a line width W197 in the direction Y corresponding to theline width W3 and a width W215 in the direction X corresponding to theline width W4. The diode layer 195 functions as a rectification elementD311 j, and the resistance change layer 197 functions as a resistancechange element R311 j.

The width W177 in the direction Y of the resistance change layer 177 inthe lower memory cell MC111 j is wider than the width W197 in thedirection Y of the resistance change layer 197 in the memory cell MC311j. Also, the width W206 in the direction X of the resistance changelayer 177 in the lower memory cell MC111 j is wider than the width W215in the direction X of the resistance change layer 197 in the uppermemory cell MC311 j.

The upper memory cell MC311 j and the lower memory cell MC111 j aredisposed on an underlying region 172.

A method for producing the nonvolatile semiconductor memory device 100 jis different from the first embodiment as shown in FIGS. 11, 12, and 10.

In the step shown in FIG. 3B, an etching processing is performed in sucha manner as to etch with a width of plural fin-like members FIN1 inaccordance with the line width W1. Other features are the same as thefirst embodiment.

In the step shown in FIGS. 11A and 11B, a conduction layer (not shown)is deposited on the plural fin-like members FIN1. After that, theconduction layer and the plural fin-like members are subjected to theetching. Thus, the conduction layer is divided into plural line patterns181 corresponding to the bit line BL11 j and the like. Simultaneously,the plural fin-like members FIN1 are divided into plural memory cellswhich are two-dimensionally aligned in the direction X and the directionY. More specifically, a memory cell array in which plural memory cellsMC111 j to MC122 j are aligned in matrix in a lower memory layer MC1 jis formed (see FIG. 9B). After that, an interlayer insulation film 209is embedded between the adjacent memory cells of MC111 j to MC122 j.

In the step shown in FIGS. 12A and 12B, an insulation film 192 coveringthe plural line patterns 181 and the interlayer insulation film 209 isformed.

In the step shown in FIGS. 10A and 10B, a multilayer film shown in FIG.3A is formed on the insulation film, and the multilayer film issubjected to an etching processing in similar manner to a mannerdescribed above to form plural memory cells each having a large widthcorresponding to the line widths W3 and W4. More specifically, a memoryarray in which plural memory cells MC311 j to MC344 j are aligned inmatrix in an upper memory layer MC3 j is formed (see FIG. 9A).Simultaneously, plural line patterns 220 corresponding to the bit linesBL21 j and the like are formed. After that, an interlayer insulationfilm 200 is embedded between the adjacent memory cells of MC311 j toMC344 j.

In order to clarify the effect according to the third embodiment,results of evaluation conducted on a relationship between a cell size(width in planar direction of resistance change layer) and dataretention will be described by using FIGS. 13A and 13B.

As shown in FIG. 13A, two different memory arrays were prepared. Morespecifically, the two memory arrays in each of which plural memory cellshaving a cell size CS1 or CS2 are aligned were prepared, and dataretention after setting (after lowering resistance) was evaluated.

More specifically, the cell size was decided so as to satisfy:

CS2=CS1×6. After an elapse of a predetermined time period, a proportionof bits (memory cells) having good data retention was evaluated in eachof the memory cell arrays. As a reference for deciding whether or notthe data retention was good, a resistance change ratio of 50% or morewas used. More specifically, as shown in FIG. 13B, the bits distributedbelow the reference line were decided to be NG. As a result, it wasconfirmed that there is a tendency that a proportion of bits having gooddata retention is increased along with an increase in cell size ofCS1→CS2. From the findings, it was confirmed that the fluctuation indata retention between the upper memory cell and the lower memory cellis reduced when the width in planar direction of the resistance changelayer 177 in the lower memory cell MC111 j for which the larger numberof heating processes is performed is larger than the width in planardirection of the resistance change layer 197 in the upper memory cellMC311 j for which the smaller number of heating processes is performed.

As described above, in the third embodiment, the width in the planardirection of the resistance change layer in the lower memory cell iswider than the width in the planar direction of the resistance changelayer in the upper memory cell. With such configuration, it is possibleto reduce the fluctuation in data retention between the upper memorycell and the lower memory cell by reducing the fluctuation in dataretention due to the difference in number of heating processes.

In the nonvolatile semiconductor memory device 100 j, the cell size(width in planar direction of resistance change layer) may be increased(widened) stepwise from the upper memory layer to the lower memorylayer. For example, in the configuration shown in FIG. 1A, cell size ofmemory cell in memory layer MC4<cell size of memory cell in memory layerMC3<cell size of memory cell in memory layer MC2<cell size of memorycell in memory layer MC1 may be satisfied.

With such configuration, it is possible to reduce in the stepwise mannerthe fluctuation in data retention due to heat history differences amongdiode formations in the layers.

Alternatively, the first embodiment and the third embodiment may becombined. More specifically, a film thickness of the resistance layer inthe lower memory cell is increased to be larger than a film thickness ofthe resistance change layer in the upper memory cell, and the width inthe planar direction of the resistance change layer in the lower memorycell is widened to be larger than the width in the planar direction ofthe resistance change layer in the upper memory cell.

Alternatively, the second embodiment and the third embodiment may becombined. More specifically, the composition of the resistance changelayer in the lower memory cell has a smaller degree of deterioration indata retention due to heat than that of the composition of theresistance change layer in the upper memory cell, and the width in theplanar direction of the resistance change layer in the lower memory cellis widened to be larger than the width in the planar direction of theresistance change layer in the upper memory cell.

Alternatively, all of the first to the third embodiments may becombined.

While certain embodiments have been described, these embodiments havebeen presented by way of example only, and are not intended to limit thescope of the inventions. Indeed, the novel embodiments described hereinmay be embodied in a variety of other forms; furthermore, variousomissions, substitutions and changes in the form of the embodimentsdescribed herein may be made without departing from the spirit of theinventions. The accompanying claims and their equivalents are intendedto cover such forms or modifications as would fall within the scope andspirit of the inventions.

1. A nonvolatile semiconductor memory device comprising: a semiconductorsubstrate; a first line disposed on the semiconductor substrate; a firstmemory cell disposed on a side opposite to the semiconductor substratewith respect to the first line; a second line intersecting with thefirst line via the first memory cell; a second memory cell disposed on aside opposite to the semiconductor substrate with respect to the secondline; and a third line intersecting with the second line via the secondmemory cell, wherein the first memory cell has a first resistance changelayer and a first rectification layer, the second memory cell has asecond resistance change layer and a second rectification layer, and acomposition of the first resistance change layer is different from acomposition of the second resistance change layer.
 2. The nonvolatilesemiconductor memory device according to claim 1, wherein a degree ofdeterioration in data retention due to heat of the composition of thefirst resistance change layer is smaller than a degree of deteriorationin data retention due to heat of the composition of the secondresistance change layer.
 3. The nonvolatile semiconductor memory deviceaccording to claim 2, wherein the first resistance change layer isformed of a carbon-based material, and the second resistance changelayer is formed of a metal oxide.
 4. The nonvolatile semiconductormemory device according to claim 3, wherein the carbon-based materialcomprises a material containing SiC as a main component or a materialcontaining a carbon nanotube as a main component, and the metal oxidecomprises a material containing, as a main component, at least oneselected from a group consisting of ZnMn₂O₄, NiO, HfO, TiO₂, SrZrO₃, andPr_(0.7)Ca_(0.3)MnO₃.
 5. The nonvolatile semiconductor memory deviceaccording to claim 1, further comprising: a third memory cell disposedon a side opposite to the semiconductor substrate with respect to thethird line; and a fourth line intersecting with the third line via thethird memory cell, wherein the third memory cell has a third resistancechange layer and a third rectification layer, the composition of thefirst resistance change layer, the composition of the second resistancechange layer, and a composition of the third resistance change layer aredifferent from one another.
 6. The nonvolatile semiconductor memorydevice according to claim 5, wherein a degree of deterioration in dataretention due to heat of the composition of the first resistance changelayer is smaller than a degree of deterioration in data retention due toheat of the composition of the second resistance change layer, and adegree of deterioration in data retention due to heat of the compositionof the second resistance change layer is smaller than a degree ofdeterioration in data retention due to heat of the composition of thethird resistance change layer.
 7. The nonvolatile semiconductor memorydevice according to claim 6, wherein the first resistance change layeris formed from a second carbon-based material, the second resistancechange layer is formed from a first carbon-based material, and the thirdresistance change layer is formed from a metal oxide.
 8. The nonvolatilesemiconductor memory device according to claim 7, wherein the secondcarbon-based material comprises a material containing a carbon nanotubeas a main component, the first carbon-based material comprises amaterial containing SiC as a main component, and the metal oxidecomprises a material containing, as a main component, at least oneselected from the group consisting of ZnMn₂O₄, NiO, HfO, TiO₂, SrZrO₃,and Pr_(0.7)Ca_(0.3)MnO₃.
 9. The nonvolatile semiconductor memory deviceaccording to claim 1, wherein a film thickness of the first resistancechange layer is larger than a film thickness of the second resistancechange layer.
 10. A nonvolatile semiconductor memory device comprising:a semiconductor substrate; a first line disposed on the semiconductorsubstrate; a first memory cell disposed on a side opposite to thesemiconductor substrate with respect to the first line; a second lineintersecting with the first line via the first memory cell; a secondmemory cell disposed on a side opposite to the semiconductor substratewith respect to the second line; and a third line intersecting with thesecond line via the second memory cell, wherein the first memory cellhas a first resistance change layer and a first rectification layer, thesecond memory cell has a second resistance change layer and a secondrectification layer, and a film thickness of the first resistance changelayer is larger than a film thickness of the second resistance changelayer.
 11. The nonvolatile semiconductor memory device according toclaim 10, further comprising: a third memory cell disposed on a sideopposite to the semiconductor substrate with respect to the third line;and a fourth line intersecting with the third line via the third memorycell, wherein the third memory cell has a third resistance change layerand a third rectification layer, and a film thickness of the secondresistance change layer is larger than a film thickness of the thirdresistance change layer.
 12. A nonvolatile semiconductor memory devicecomprising: a semiconductor substrate; a first line disposed on thesemiconductor substrate; a first memory cell disposed on a side oppositeto the semiconductor substrate with respect to the first line; a secondline intersecting with the first line via the first memory cell; a thirdline disposed on a side opposite to the semiconductor substrate withrespect to the second line; a second memory cell disposed on a sideopposite to the semiconductor substrate with respect to the third line;and a fourth line intersecting with the third line via the second memorycell, wherein the first memory cell has a first resistance change layerand a first rectification layer, the second memory cell has a secondresistance change layer and a second rectification layer, and a width ofthe first resistance change layer in a planar direction which isparallel to a surface of the semiconductor substrate is larger than awidth of the second resistance change layer in a planar direction whichis parallel to the surface of the semiconductor substrate.
 13. Thenonvolatile semiconductor memory device according to claim 12, furthercomprising an insulation film insulating the second line and the thirdline from each other, wherein a width of each of the first line and thesecond line is larger than a width of each of the third line and thefourth line.
 14. The nonvolatile semiconductor memory device accordingto claim 13, further comprising: a fifth line disposed adjacent to thethird line via an interlayer insulation film on a side opposite to thesemiconductor substrate with respect to the second line; a third memorycell disposed adjacent to the second memory cell via the interlayerinsulation film on a side reverse to the semiconductor substrate withrespect to the fifth line; and a sixth line intersecting with the fifthline via the third memory cell, wherein a width of each of the firstline and the second line is equivalent to a sum of the width of each ofthe third line and the fourth line, a width of each of the fifth lineand the sixth line, and a space between the third line and the fifthline.
 15. The nonvolatile semiconductor memory device according to claim14, wherein the third memory cell has a third resistance change layerand a third rectification layer; and a width of the second resistancechange layer in a planar direction which is parallel to the surface ofthe semiconductor substrate is equivalent to a width of the thirdresistance change layer in a planar direction which is parallel to thesurface of the semiconductor substrate.
 16. The nonvolatilesemiconductor memory device according to claim 12, further comprising: aseventh line disposed on a side opposite to the semiconductor substratewith respect to the fourth line; a fourth memory cell disposed on a sideopposite to the semiconductor serrate with respect to the seventh line;and an eighth line intersecting with the seventh line via the fourthmemory cell, wherein the fourth memory cell has a fourth resistancechange layer and a fourth rectification layer, and a width of the secondresistance change layer in a planar direction which is parallel to thesurface of the semiconductor substrate is larger than a width of thefourth resistance change layer in a planar direction which is parallelto the surface of the semiconductor substrate.
 17. The nonvolatilesemiconductor memory device according to claim 12, wherein a filmthickness of the first resistance change layer is larger than a filmthickness of the second resistance change layer.
 18. The nonvolatilesemiconductor memory device according to claim 12, wherein a compositionof the first resistance change layer is different from a composition ofthe second resistance change layer.
 19. The nonvolatile semiconductormemory device according to claim 18, wherein a degree of deteriorationin data retention due to heat of the composition of the first resistancechange layer is smaller than a degree of deterioration in data retentiondue to heat of the composition of the second resistance change layer.20. The nonvolatile semiconductor memory device according to claim 18,wherein a film thickness of the first resistance change layer is largerthan a film thickness of the second resistance change layer.